An electric power steering device, which detects a steering torque and imparts an assist torque (steering assist torque) based on the steering torque, is known as a general steering device. A steering control device for controlling drive of the electric power steering device of this type controls the steering torque sensed by the driver steering the steering wheel, and thus constitutes an important factor forming steering feeling of a vehicle.
On this occasion, a road surface reaction force torque is a torque acting in a direction of returning the steering wheel to a neutral point by a tire lateral force acting between the road surface and steered wheels. The road surface reaction force torque may be small depending on a layout of the steering system, specifications of tires, and the like on some vehicles.
When the road surface reaction force torque is small in this way, the road surface reaction force torque cannot overcome a friction torque, which is relatively large. As a result, the road surface reaction force torque acting in the direction in which the steering wheel is returned to the neutral point becomes insufficient during the steering by the driver. It is thus necessary for the driver to intentionally apply a return torque to the steering wheel, thereby returning the steering wheel to the neutral point.
Moreover, when the road surface reaction force torque is small, it is difficult for the driver to sense, based on a reaction force transmitted to the hands of the driver, whether the steering wheel is at the neutral point, or the steering wheel is displaced from the neutral point and the vehicle is thus turning. It is thus necessary for the driver to visually confirm the steering wheel position so that the vehicle is stably controlled to travel straight. As described above, in a case where the road surface reaction force torque is small, there has been a problem that the steering feeling decreases.
In order to address this problem, an electronic control unit (ECU) drives a motor so as to impart a steering wheel return torque based on a steering angle and a direction of a steering angular velocity to a steering system in a conventional device described in Patent Literature 1, for example.
Moreover, an ECU estimates a rack shaft force from a steering angle based on a vehicle model, and carries out assist control based on the estimated rack shaft force value in a conventional device described in Patent Literature 2, for example.
Further, an ECU calculates an assist torque by using a result obtained by subtracting a steady-state response component, which can be mathematically calculated, from a steering response characteristic represented as a vehicle model out of response characteristics of the steering torque with respect to a steering angle in a conventional device described in Patent Literature 3, for example. This calculated assist torque cancels a characteristic of a high frequency component (non-steady state response component) out of the response characteristics of the steering torque with respect to the steering angle.
Moreover, an ECU compensates, in phase, a detected or estimated self-aligning torque in order to compensate a cornering force affected by a suspension geometry and a transmission delay of a self-aligning torque dynamic characteristic, and corrects a steering assist command value in a conventional device described in Patent Literature 4), for example.